1. Field of the Invention
The present invention relates to a cathode for a lithium secondary battery and a process for producing the same.
2. Description of the Related Art
In recent years, portable information instruments, such as a personal computer, a portable telephone and an information terminal, have been widely used. Since these instruments have various multimedia functions, the secondary battery used for such instrument as power supply is demanded to be small and light having a large capacity, namely, to have a high energy density. In this regard, aqueous secondary batteries, such as a lead-acid battery and a nickel-cadmium battery conventionally used, are not sufficient. Lithium secondary batteries which can attain a higher energy density, especially the lithium secondary batteries using as a cathode active material, lithium composite oxide such as lithiated cobalt dioxide, lithiated nickel dioxide, and spinel lithium manganese oxide, and as an anode active material, a carbonaceous material that can be doped/undoped with lithium ions, have been developed.
Since these lithium secondary batteries have inherently a large energy, maximum safety against abnormalities, such as an internal short circuit and an external short circuit, is required. Especially, in case of the severe safety test such as a nail penetration test or a crush test which causes the internal short circuit with local heat-generation, the charged cathode active material decomposes and releases oxygen causing another heat-generation. It is found that when the separator consisting of olefin resins such as polyethylene or polypropylene exists near the cathode electrode at the short circuit section, the separator is oxidized by the oxygen which is released from the cathode active material and that it causes s remarkable heat-generation which may result in an uncontrollable exothermic reaction.
On the other hand, in case of the separator comprising a heat-resistant resin alone such as fluororesin and the like, the shut-down function does not work, and a sufficient safety may not be kept against the internal heating accompanied by a large current charging/discharging or an external heating. Moreover, in case of the separator comprising two piled films of a heat-resistant resin and an olefin resin conventionally used, the separator becomes thick, and the amount of the active material which can be contain in a battery case decreases, and therefore the battery capacity decreases. Furthermore, the separator comprising two piled films increases the battery resistance, and results in further capacity drop in use at the large current.
It is described in JP-A 7-296847 that safety can be improved by forming a thin film of an ion-permeable polymer comprising fluororesin on a surface of a cathode. However, there is a problem that the heat resistance is not enough in case of the battery with high capacity using, for example, lithiated nickel dioxide as a cathode active material.